javascript 输入框监听事件

<div class="coupon-exchange clearfix">
        <div class="code-input">
            <input type="text" placeholder="请输入兑换码" class="input" id="couponExchangeInput"/>
            <i class="icon-clear" id="couponExchangeClear"></i>
        </div>
        <div class="btn-exchange" id="couponExchangeBtn">兑换</div>
    </div>

function couponExchange() {
　　//中文输入，输入完成后才触发事件
    var flag = true;
    $('#couponExchangeInput').on('compositionstart',function(){
        flag = false;
    });
    $('#couponExchangeInput').on('compositionend',function(){
        flag = true;
    });
    $('#couponExchangeInput').on('input', function(){
        var _this = this;
        setTimeout(function(){
            if(flag){
                var _val = $(_this).val();
                $('#couponExchangeBtn').addClass('active');
                $('#couponExchangeClear').css('display', 'block');
                if(_val == ''){
                    $('#couponExchangeBtn').removeClass('active');
                    $('#couponExchangeClear').hide();
                }
                if(strlen(_val) > 100){
                    var _cutVal = cut_str(_val, 50);
                    $(_this).val(_cutVal);
                }
            }
        },0);
    });
    $('#couponExchangeClear').click(function(){
        $('#couponExchangeBtn').removeClass('active');
        $('#couponExchangeClear').hide();
        $('#couponExchangeInput').val('').focus();
    });
    $('#couponExchangeInput').blur(function(){
        var _val = $(this).val();
        if(strlen(_val) > 100){
           var _cutVal = cut_str(_val, 50);
            $(this).val(_cutVal);
        }
    });
    $('#couponExchangeBtn').click(function(){
        if($(this).hasClass('active')){
            console.log(1);
        }
    });
}
//获取字符串的长度，一个中文占位两个字符长度
function strlen(str) {
    var len = 0;
    for (var i = 0; i < str.length; i++) {
        var c = str.charCodeAt(i);
        if ((c >= 0x0001 && c <= 0x007e) || (0xff60 <= c && c <= 0xff9f)) {
            len++;
        }
        else {
            len += 2;
        }
    }
    return len;
}
//截取字符串的长度，一个中文占位两个字符长度
function cut_str(str, len){
    var char_length = 0;
    for (var i = 0; i < str.length; i++){
        var son_str = str.charAt(i);
        encodeURI(son_str).length > 2 ? char_length += 1 : char_length += 0.5;
        if (char_length >= len){
            var sub_len = char_length == len ? i+1 : i;
            return str.substr(0, sub_len);
            break;
        }
    }
}